Method and apparatus for tempering glass sheets

Abstract

A method and an apparatus for tempering glass sheets. A glass sheet is heated to a tempering temperature and quenching is conducted by blasting cooling air to both surfaces of the glass sheet. The quenching of a top surface and a bottom surface of the glass sheet's both side portions is commenced earlier or is performed at the early stage of quenching more effectively than the quenching of a top surface and a bottom surface of the glass sheet's intermediate portion. As a result, the compression stress required for a desired tempering degree is established on both surfaces of the side portions earlier than on both surfaces of the intermediate portion. In order to achieve this, the cooling air enclosures above and below a glass sheet are provided with a subarea of weakened cooling effect.

The invention claimed is: 
     
       1. An apparatus for tempering thin planar glass sheets with a thickness of not more than 4 mm to a surface compression stress of at least 100 MPa, said apparatus comprising:
 a furnace for heating glass sheets to a tempering temperature, the furnace including a conveyor track for glass sheets, and 
 a quenching unit for cooling glass sheets, the quenching unit including a conveyor track and cooling air enclosures set above and below the quenching unit conveyor track and having cooling air blast openings in such a disposition that a cooling effect of a blast through the blast openings is directed to the top and bottom surfaces of a glass sheet across the entire width of the glass sheet moving in the quenching unit, 
 wherein the blast openings have a shortest blasting distance of not more than 30mm from the surface of the glass sheet moving on the quenching unit conveyor track and a blasting pressure of not less than 6 kPa, 
 wherein, in order to eliminate or reduce the bi-stability of a tempered flat glass sheet, at least a first cooling air enclosure above the quenching unit conveyor track includes a subarea of weakened or blocked cooling effect and at least a first cooling air enclosure below the quenching unit conveyor track includes a subarea of weakened or blocked cooling effect, 
 wherein said subareas having a weakened or blocked cooling effect as compared to the cooling effect of cooling air enclosures outside the subareas over a surface area of cooling air enclosures equivalent to the subareas, the subareas being located above and below an intermediate portion of the moving glass sheet, 
 whereby quenching of a top surface and a bottom surface of opposing side portions of the glass sheet commences earlier or more effectively than the quenching of a top surface and a bottom surface of the intermediate portion of the glass sheet, as a result of which the opposing side portions are provided, prior to the intermediate portion, with a temperature profile in the glass thickness direction which generates a tempering degree having the surface compression stress of at least 100 MPa, 
 wherein quenching of the intermediate portion is commenced on both the top and bottom surfaces of the glass sheet not more than 30 cm after the opposing side portions, and 
 wherein a distance from the furnace to below the first cooling air enclosure is not more than a thickness of an end insulation of the furnace plus 300 mm and the glass sheet has a motion speed of at least 300 mm/s, whereby a transfer time for a leading edge of the glass sheet from the end insulation of the furnace to below the first cooling air enclosures is not more than 1 s. 
 
     
     
       2. An apparatus according to  claim 1 , wherein the subareas of weakened cooling effect are located at a start of the quenching unit over a distance of the first 0-60 cm and the subareas have a length in the motion direction of glass which is not less than a diameter of a cooling air blast opening and not more than 60 cm. 
     
     
       3. An apparatus according to  claim 1 , wherein on the intermediate portion, which is included in the glass sheet advancing in the quenching unit and whose width is at least 50% of the width of the glass sheet, quenching is commenced on both the top and bottom surfaces of the glass sheet at least 4 cm after the opposing side portions. 
     
     
       4. An apparatus according to  claim 1 , wherein, in the motion direction of the glass sheet downstream of the subareas of weakened cooling effect, the cooling arrangement and a cooling effect generated thereby are substantially identical across the entire width of the glass sheet. 
     
     
       5. An apparatus according to  claim 1 , wherein, in the subarea of weakened cooling effect, the blast openings have a total surface area which is smaller than the total surface area of blast openings included in a similar size area of the cooling air enclosure outside the subarea, and that the reduction of surface area is effected by reducing a diameter of the blast openings and/or by reducing a number of blast openings and/or by closing some blast openings completely or partially. 
     
     
       6. An apparatus according to  claim 1 , wherein the subareas having weakened or blocked cooling effect are completely or partially effected by means of a barrier placed in the way of blasting jets discharging from the blast openings. 
     
     
       7. An apparatus according to  claim 1 , wherein the subareas having weakened or blocked cooling effect are effected by increasing a blasting distance between the blast openings in the subareas and the glass sheet as compared to a blasting distance outside the subareas. 
     
     
       8. An apparatus according to  claim 1 , wherein the intermediate portion of the glass sheet has a width of at least 5 cm, each of the opposing side portions of the glass portion has width of at least 5 cm, and the subareas of weakened cooling effect have a width of at least 20% with respect to the width of the glass sheet. 
     
     
       9. An apparatus according to  claim 1 , wherein a heat transfer coefficient produced by blasting on the opposing side portions of a glass sheet in a lateral direction of the quenching unit on both sides of the subareas are substantially equal to a heat transfer coefficient of the rest of the quenching unit on the glass sheet, and a heat transfer coefficient produced by blasting in the subareas on the intermediate portion of a glass sheet is on average at least 20% lower than that. 
     
     
       10. An apparatus according to  claim 1 , wherein, in a direction perpendicular to a motion direction of a glass sheet, the subareas lie substantially symmetrically in the middle of the glass sheet and the subareas are substantially consistent on both the top and the bottom surfaces of the glass sheet. 
     
     
       11. An apparatus according to  claim 1 , wherein a cooling effect in a lateral direction of the glass sheet does not change abruptly at a boundary of the subareas, but the cooling effect is adapted to be of a gradually changing type.